GB1437075A - Gas detectors - Google Patents

Abstract

1437075 Measuring gas concentration; resistance bridge NATIONAL RESEARCH DEVELOPMENT CORP 25 June 1973 [30 June 1972] 30762/72 Headings G1N and G1U The concentration of a test gas in a gas mixture is determined by the change in temperature of a heated catalyst, and is measured by the electrical power required to maintain the temperature of the catalyst at a constant value. As shown, Fig. 1, a bead 14 of a refractory material containing a catalytic substance and a platinum wire coil is supported in an enclosure 11, which is open to an atmosphere with a contaminant gas whose concentration is to be determined through a first aperture 12, a filter 13 and an adjustable iris diaphragm 24, and is fed through a second aperture 20 with methanol. The methanol vapour reacts with the oxygen of the atmosphere to raise the temperature of the bead 14 by an amount dependant on the test gas concentration. The temperature is measured by the resistance of the platinum wire in the bead 14, when connected in a resistance bridge network ABCD Fig. 3. The bridge unbalance at BD is fed back via transistors 36, 38 to vary the bridge energizing voltage at A.C. to maintain the temperature and resistance of bead 14 and the unbalance voltage at a constant value. The voltage across the bead 14 is then indicative of the concentration of the contaminant gas.